1. Field of the Invention
The invention relates to a sensor for measuring a physical quantity which causes a non-reciprocal effect in an optical ring resonatormeasuring quantity.
In particular, the invention relates to a sensor which is based on the fact that the measuring quantity causes non-reciprocal effects on a ring resonator. Such non-reciprocal effects are, for example, the Sagnac-effect or the Faraday-effect. The Sagnac-effect is, for example, used for producing "laser gyros".
2. Description of the Prior Art
From DE-OS 37 12 815 or EP-PS 0 290 723 (substantially with the same teaching) a measuring instrument for rotation rates making use of the Sagnac-effect is known, which contains a ring resonator and a laser. Therein, the ring resonator forms a part of the resonator cavity of the laser. Thereby, no precautions have to be taken to achieve optical insulation between laser and ring resonator. A semiconductor laser serves as laser. Therein, different embodiments of the ring resonator having mirrors, integrated optics or fiber optics are described. A phase modulation of the laser is effected by periodic variation of the light path contained in the ring resonator. The light path is adjusted such that the frequency of the laser light corresponds to the resonance frequency of the ring resonator for one direction of circulation of the light. The derivative of the intensity of the light circulating in the other direction of circulation with respect to the phase, practically the a.c. component of the modulated light, then is indicative of the rotation rate. In DE-OS 37 12 815 it is also proposed to modulate the laser directly through the control of an injection current. In practice, however, the difficulty arises, that the modulation capability of the laser is reduced by the feedback of the light from the external oscillator.
A fiber gyro having a passive ring resonator is described in a paper by Carroll, Coccoli, Cardarelli and Coate. "The Passive Resonator Fiber Optic Gyro and Comparison to the Interferometer Fiber Gyro", in "SPIE", vol. 719 Fiber Optic Gyros: 10th Anniversary Conference (1986), 169-177. The light from the laser is modulated by a phase modulator for generating sidebands and is guided in two fiber branches. An acousto-optical frequency shifter is located in each of the two fiber branches. From the two fiber branches the light is coupled into a fiber ring with opposite direction of circulation. Correspondingly, the light which has passed the fiber ring in one or the other direction of circulation is coupled out by couplers and guided to photoelectric detectors. The detector signals are applied to signal processing means. These control the laser frequency on one hand, and one of the frequency shifters on the other hand. Thus, when a rotation rate occurs, the frequency of the laser light is kept in resonance with one direction of circulation of the ring resonator formed by the fiber ring, whereas a corresponding frequency shift is effected in the other direction of circulation.
From a paper by Hollberg and Ohtsu, "Modulatable narrow-linewidth semiconductor lasers", in Appl. Phys. Lett. 53 (1988), 944-946 it is known to improve the bandwidth of a semiconductor laser by optical feedback from an external resonator cavity. Therein, it is also said that the modulation capability of the laser is reduced by such an optical feedback. Therein, it is also said that certain modulation frequencies exist which strongly affect the frequency modulation properties of the optically stabilized semiconductor laser. In certain modulation states, some or all of the modulation sidebands can be in resonance with the resonator cavity with or without carrier. In this case these sidebands return to the semiconductor laser and amplify its optical stabilization. This is particularly valid for the free spectral distance of the resonator cavity or the harmonics thereof. In this way, it is possible to modulate the laser current with a high modulation index and to generate many sidebands without disturbing the frequency stabilization and the linewidth narrowing which is achieved by the optical feedback.
In a paper by Laurent, Clairon and Breant, "Frequency Noise Analysis of Optically Self-Locked Diode Lasers" in Journal of Quantum Electronics, vol. 25 (1989), 1131-1142, it is illustrated how the frequency of an optically fed back semiconductor laser varies as a function of the frequency of the "undisturbed" semiconductor laser, and shows the strong reduction of the modulation capability.
In a paper by DeVoe and Brewer, "Laser-frequency division and stabilization", in Physical Review A, vol. 30 (1984), 2987-2889 an arrangement is illustrated, in which a laser is turned by a control circuit to a higher order of a reference cavity. The reference cavity is tuned by a second control circuit to a high frequency. Thus, the laser can be stabilized by means of the high frequency.
European patent application 0 405 831 discloses a ring resonator gyro, wherein the clockwise and counter-clockwise beams are modulated to provide each with a spectrum including a carrier and an upper and lower side band. The upper side band of one beam is kept at a resonance by a path length control loop, and the upper side band of the other beam is kept at a resonance by a rate control loop.